This invention relates to a method and apparatus for the continuous casting of metals, and particularly the casting of metal strip.
The continuous casting of thin metal strip has been employed with only limited success. By and large, prior processes for the continuous casting of metal strip have been limited to a relatively small number of alloys and products. It has been found that as the alloy content of various metals are increased, as-cast surface quality deteriorates. As a result, many alloys must be fabricated using ingot methods.
In the case of aluminum, relatively pure aluminum product such as foil can be continuously strip cast on a commercial basis. Building products can likewise be continuously strip cast, principally because surface quality in the case of such building products is less critical than in other aluminum products, such as can stock. However, as the alloy content of aluminum is increased, surface quality problems appear, and strip casting has generally been unsuitable for use in making many aluminum alloy products.
A number of strip casting machines have been proposed in the prior art. One conventional device is a twin belt strip casting machine, but such machines have not achieved widespread acceptance in the casting of many metals, and particularly metal alloys with wide freezing ranges. In such twin belt strip casting equipment, two moving belts are provided which define between them a moving mold for the metal to be cast. Cooling of the belts is typically effected by contacting a cooling fluid with the side of the belt opposite the side in contact with the molten metal. As a result, the belt is subjected to extremely high thermal gradients, with molten metal in contact with the belt on one side and a water coolant, for example, in contact with the belt on the other side. The dynamically unstable thermal gradients cause distortion in the belt, and consequently neither the upper nor the lower belt is flat. The product thus produced has areas of segregation and porosity as described below.
Leone, in the Proceedings Of The Aluminum Association, Ingot and Continuous Casting Process Technology Seminar For Flat Rolled Products, Vol. II, May 10, 1989, said that severe problems develop if belt stability and reasonable heat flow are not achieved. In the first place, if any area of the belt distorts after solidification of the molten metal has begun and strip shell coherency has been reached, the resulting increase in the gap between the belt and the strip in the distorted region will cause strip shell reheating, or, at least, a locally reduced shell growth rate. That, in turn, gives rise to inverse segregation in the strip which generates interdendritic eutectic exudates at the surface. Moreover, in severe cases with medium and long freezing range alloys, liquid metal is drawn away from a distorted region to feed adjacent, faster solidifying portions of the strip. That in turn causes the surface of the strip to collapse and forms massive areas of shrinkage porosity in the strip which can crack on subsequent rolling or produce severe surface streaks on the rolled surface.
As a result, twin belt casting processes have not generally achieved acceptance in the casting of alloys for surface-critical applications, such as the manufacturing of can stock. Various improvements have been proposed in the prior art, including preheating of the belts as described in U.S. Pat. Nos. 3,937,270 and 4,002,197, continuously applied and removed parting layers as described in U.S. Pat. No. 3,795,269, moving endless side dams as described in U.S. Pat. No. 4,586,559 and improved belt cooling as described in U.S. Pat. Nos. 4,061,177, 4,061,178 and 4,193,440. None of those techniques has achieved widespread acceptance either.
An additional approach to continuous belt casting of steel is described in U.S. Pat. No. 4,561,487 utilizing a pair of counter-rotating belts in which one is chilled while it is not in contact with the metal being cast. Thereafter, a supply of molten steel is supplied to the surface of the belt just before the belt passes downwardly and around a supporting pulley and the metal being cast is passed between the belts. While the approach taken in that patent may avoid the thermal distortion affects caused by large temperature gradients when a cooling fluid is supplied to one side of the belt and the other side of the belt is in contact with hot metal, it presents other problems. The supply of the molten metal to the belt just as it passes around a supporting pulley means that the molten metal must be cooled very quickly; otherwise, molten metal will flow off the belt into the area surrounding the equipment, representing a hazard to workers. In addition, the '487 patent casts the molten metal on a single belt, and uses the second belt only as a "hugger" belt to maintain the cast ribbon in contact with the chilled belt.
Other attempts at belt casting approaches are described in U.S. Pat. No. 3,432,293 and published European Application No. 0,181,566. In the techniques described by both publications, a cooling liquid is applied to the opposite side of a belt on which a metal is cast both while the belt is not in contact with the metal and while it is in contact with the metal. Thus, neither recognizes the concept that the heat transmitted to the belt from the molten metal is substantially removed by application of a cooling fluid at a time when the belt is out of contact with the metal being cast to avoid formation of large thermal gradients.
Another continuous casting process that has been proposed in the prior art is that known as block casting. In that technique, a number of chilling blocks are mounted adjacent to each other on a pair of opposing tracks. Each set of chilling blocks rotates in the opposite direction to form therebetween a casting cavity into which a molten metal such as an aluminum alloy is introduced. The liquid metal in contact with the chilling blocks is cooled and solidified by the heat capacity of the chilling blocks themselves. Block casting thus differs both in concept and in execution from continuous belt casting. Block casting depends on the heat transfer which can be effected by the chilling blocks. Thus, heat is transferred from the molten metal to the chilling blocks in the casting section of the equipment and then extracted on the return loop. Block casters require precise dimensional control to prevent flash (i.e. transverse metal fins) caused by small gaps between the blocks. Such flash causes sliver defects when the strip is hot rolled. As a result, good surface quality is difficult to maintain. Examples of such block casting processes are set forth in U.S. Pat. Nos. 4,235,646 and 4,238,248.
Another technique which has been proposed in continuous strip casting is the single drum caster. In single drum casters, a supply of molten metal is delivered to the surface of a rotating drum, which is internally water cooled, and the molten metal is dragged onto the surface of the drum to form a thin strip of metal which is cooled on contact with the surface of the drum. The strip is frequently too thin for many applications, and the free surface has poor quality by reason of slow cooling and micro-shrinkage cracks. Various improvements in such drum casters have been proposed. For example, U.S. Pat. Nos. 4,793,400 and 4,945,974 suggest grooving of the drums to improve surface quality; U.S. Pat. No. 4,934,443 recommends a metal oxide on the drum surface to improve surface quality. Various other techniques are proposed in U.S. Pat. Nos. 4,771,819, 4,979,557, 4,828,012, 4,940,077 and 4,955,429.
Another approach which has been employed in the prior art has been the use of twin drum casters, such as in U.S. Pat. Nos. 3,790,216, 4,054,173, 4,303,181, or 4,751,958. Such devices include a source of molten metal supplied to the space between a pair of counter-rotating, internally cooled drums. The twin drum casting approach differs from the other techniques described above in that the drums exert a compressive force on the solidified metal, and thus effect hot reduction of the alloy immediately after freezing. While twin drum casters have enjoyed the greatest extent of commercial utilization, they nonetheless suffer from serious disadvantages, not the least of which is an output substantially lower than that achieved in many prior art devices described above. Once again, the twin drum casting approach, while providing acceptable surface quality in the casting of high purity aluminum (e.g. foil), suffers from poor surface quality when used in the casting of aluminum with high alloy content and wide freezing range. Another problem encountered in the use of twin drum casters is center-line segregation of the alloy due to deformation during solidification.
There is thus a need to provide an apparatus and method for continuously casting thin metallic strip at high speeds and improved surface quality as compared to methods currently employed.
In co-pending application Ser. No. 07/902,997, filed Jun. 23, 1992 now abandoned, the disclosure of which is incorporated herein by reference, there is described a method and apparatus where the continuous casting of metal strip, and particularly metal strip formed form highly alloyed aluminum, which overcomes many limitations of the prior art disclosed above. In the method and apparatus there described, uses made of the heat sink capabilities of the belts in a substantially horizontal molding zone in which substantially all of the heat transmitted to the belts from the metal being cast is removed from the belts while the belts are out of contact with the metal being cast. In that way, the method and apparatus described in the foregoing application substantially minimizes the formation of thermal gradients over the thickness of the belts which caused distortion of belts used in the prior art.
In co-pending application Ser. No. 08/173,663, filed concurrently herewith, there is described a method and apparatus for the continuous casting of metal strip which represents a significant improvement in such casting operation. In the method and apparatus there described, each of the twin belts is passed around a pulley, thereby defining a curved surface of the belt followed by a substantial flat surface, both of which define the molding zone between the belts. In accordance with the concepts of that invention, the molten metal is supplied to the molding zone between the belts to the curved surfaces of each of the belts. In the preferred embodiment of that invention, the molten metal solidifies in the molding zone by the time it reaches the nip of the pulleys supporting the belts, that is to say, the point along a line passing through the axis of the pulleys perpendicularly to the belts. As described in that application, it has been found that supplying the molten metal to a curved surface to each of the belts in contact with their respective pulleys minimizes thermal distortion by providing increased stability and minimizes distortion of the belt preceding the nip of the supporting pulleys.
It is an object of the present invention to provide further improvements in which the molten metal is supplied on the curvature of the belts and solidifies substantially before the nip of the entry pulleys while restraining the nip of the pulleys from being displaced by solidified metal. It has been found that the positive control of the gap between the nip of the entry pulleys, combined with solidification prior to the nip, cause a compressive force to be exerted on the cast strip at the nip which in turn serves to even further minimize distortion of the belts and to reduce cracking of the metal.
It is a more specific object of the invention to provide an apparatus and method for the continuous casting of thin metallic strip which provides improved surface quality even when processing metals such as aluminum with high alloy content.
These and other objects and advantages of the invention appear more fully hereinafter from a detailed description of the invention.